1. Field of the Invention
The present invention relates to an electronic ballast for operating at least one gas discharge lamp wherein the gas discharge lamp is supplied with an alternating voltage and wherein the lamp filaments are preheated.
2. Description of the Related Art
Such a ballast is known from EP-A1-0 490 329.
As in the case of other lamps as well, in the case of gas discharge lamps on account of the phenomena of wear of the heater filaments at the end of the life of the gas discharge lamp the effect that occurs is one where the lamp electrodes wear unevenly over time, that is, the erosion of the emitting layers on the lamp electrodes is different. On account of the differing wear of the lamp electrodes, differences result in the emitting power of the two lamp electrodes.
FIG. 5 shows the consequences of this effect with reference to the current i.sub.L that is fed to the gas discharge lamp. It can be seen from FIG. 5 that a higher current flows in the one direction than in the other so that the time characteristic i.sub.L (t) has an excess of one half-wave (in FIG. 5 the positive half-wave) . As a result of the different erosion of the two lamp electrodes, asymmetries thus result that not only give rise to comparatively great light-flickering at the end of the life of the gas discharge lamp, but even in the extreme case also only permit operation of the gas discharge lamp during one half-wave (in FIG. 5 during the positive half-wave). In this case, the gas discharge lamp acts as a rectifier so that the previously described effect is termed a "rectification effect".
The work function for the electrons is higher at that electrode which has worn away to a greater extent in the course of time than at the other electrode which has worn away to a lesser extent. The minimum energy required to draw an electron out of a metal, in the present case out of the lamp electrode, is generally termed the work function. The dipole layer at the surface of the metal, that is, the lamp electrode, is then an important factor in determining the work function. The electrode that has worn away to a greater extent and which has a higher work function for the electrons than the electrode which has worn away to a lesser extent consequently heats up to a greater extent when the gas discharge lamp is put into operation than the opposing electrode. The increase in temperature in the electrode can be so great, in particular in the case of lamps with a small diameter, that portions of the glass lamp bulb can melt. In order to avoid the risk of an accident resulting from the increase in temperature of the glass lamp bulb, consequently it is necessary to identify the rectification effect and, if applicable, switch off the gas discharge lamp or reduce its power input, in which case there are already mandatory standards for monitoring the previously described uneven emission of the lamp electrodes.
As has already been mentioned above, the rectification effect manifests itself in asymmetry of the lamp current i.sub.L flowing by way of the gas discharge path of the lamp. one possibility for identifying the rectification effect is therefore to monitor the lamp current flowing by way of the gas discharge path of the lamp, in which case with this method it is certainly possible to identify differences in emission of the lamp electrodes directly, but the evaluation of these emission differences and also the translation of this identification process into a monitoring circuit arrangement that is designed as an integrated circuit, in particular as an application specific circuit (ASIC), are problematic. As an alternative to this, it is also possible to identify the rectification effect by monitoring the lamp voltage, since the asymmetries occurring in the lamp current are transferred to the lamp voltage. If, for example, the monitored lamp voltage exceeds a specific limiting value in one direction as a consequence of the asymmetrical emission of the lamp electrodes, the gas discharge lamp is switched off. In the case of this identification process, however, it is disadvantageous that the sensitivity of this method is limited, since in the case of a fault, that is, if the rectification effect occurs, the peak value of the lamp voltage that is detected is merely 60% higher than its value in the normal operating case. Moreover, even when the gas discharge lamp is dimmed, the lamp voltage is changed so that on account of the dimming of the gas discharge lamp and on account of the lamp voltage that rises in a corresponding manner as a result, it may possibly be concluded by mistake that the rectification effect is present in the gas discharge lamp. Furthermore, it would be desirable to use the changing arithmetical mean value of the monitored circuit variable for the detection of the rectification effect. This is not a possibility, however, when monitoring the lamp voltage, since--as already described--in the case of a fault the peak value of the lamp voltage is merely increased by 60% so that the increase in the mean value of the lamp voltage is not sufficient to detect the rectification effect in a sufficiently precise manner. All in all, therefore, the detection of the rectification effect by monitoring the lamp voltage is problematic.
In the case of the electronic ballast known from EP-A1-0 490 329 belonging to the applicant, a first resistor is connected in series with the primary winding of the filament-heating transformer. The current flowing through the primary winding and the first resistor generates a voltage at the resistor, which voltage is proportional to the current flowing through the heater filaments of the lamp. The voltage drop across the first resistor is evaluated by a control and regulating circuit arrangement in order to detect overvoltage or undervoltage. Identification of a rectification effect is not, however, described in this publication.
Identification of a rectification effect is, however, described in U.S. Pat. No. 5,023,516. For this purpose, a monitoring circuit arrangement is provided that comprises a series circuit arrangement consisting of two resistors and an inductor, with the series circuit arrangement being connected in parallel with a gas discharge lamp that is to be monitored. A thyristor, which is coupled to the inverter of the ballast, acts at the interconnection point between the one resistor and the inductor and thus evaluates the voltage dropping across the one resistor for the purpose of identifying the rectification effect. As soon as the voltage, which drops across the one resistor and which is proportional to the current flowing by way of the one resistor, has reached a specific limiting value, the thyristor is activated and consequently the inverter is switched off. The known monitoring circuit arrangement, however, only detects the presence of a rectification effect in one direction of polarity of the voltage dropping across the resistor.